102 Feedback City

[ToneMerc]

Tony, I never seen a high plate skyliner with 100K level controls, sans snubbers and V1 LNFB cause so much trouble...

TM

[talbany]

iPhone error

[Max]

The main reason for my labeling it 102 instead of 124 was because I used the schematic/layout for my build, minus the changes of coarse..

Tony, thanks for explaining. I understand. Your layout of #102 just has been your starting point.

124 is a low plate with different bypass caps and a different GNFB circuit A 250k drive and 340k od trimmer as well.. I believe these changes in 124 to have a greater impact on the timbre/sonic qualities than the changes made to my amp..

IMO it could perhaps be discussed rather controversially, if "these changes in 124... have a greater impact on the timbre/sonic qualities than the changes made to my amp.." IMO it could perhaps be argued, that the omission of snubbers and the V1 LNFB, a different OT, a different power supply (PT and filtering) and all your other changes may perhaps have at least the same amount of "impact on the timbre/sonic qualities" as the difference between a high plate and a low plate configuration and the other differences between the skyline version of #124 and RF's #102.

Thanks again and a nice weekend,

Max

[talbany]

The main reason for my labeling it 102 instead of 124 was because I used the schematic/layout for my build, minus the changes of coarse..

Tony, thanks for explaining. I understand. Your layout of #102 just has been your starting point.

124 is a low plate with different bypass caps and a different GNFB circuit A 250k drive and 340k od trimmer as well.. I believe these changes in 124 to have a greater impact on the timbre/sonic qualities than the changes made to my amp..

IMO it could perhaps be discussed rather controversially, if "these changes in 124... have a greater impact on the timbre/sonic qualities than the changes made to my amp.." IMO it could perhaps be argued, that the omission of snubbers and the V1 LNFB, a different OT, a different power supply (PT and filtering) and all your other changes may perhaps have at least the same amount of "impact on the timbre/sonic qualities" as the difference between a high plate and a low plate configuration and the other differences between the skyline version of #124 and RF's #102.

Thanks again and a nice weekend,

Max

Max
You are right it could be debated!!.. I admit I am a high plate Junkie

With Deepest Respect!!

Tony

[lovetone]

Hey Tony

Have you had a chance to carry out any more testing on your amp?

Cheers Geoff

[talbany]

The whole thing is that the amp appears to be on the verge on instability. Dave Funk talked about deliberately building instability in his amps via lead dress/layout. I had encountered that with my early Fender conversion amps back in the day; it started off as a problem, because I would get this "whistling sound" in OD mode. Once I found the problem area I could "fix things" and was left with an amp that would also take off almost all by itself.

To me, however, that didn't readily translate into the Dumble lead dress too much since it seems to be pretty much a "left to right" type of layout, such that there isn't any "obvious" in-phase coupling anywhere. Or, is there, and we have been missing this all these years? Dumble does hint at that in the 80s GP interview, where he says very small distances in the lead dress are crucial: "A distance of half a centimeter makes a big difference in the way something sounds. It's a science involved with what's called circuit constants."

I have absolute faith that you will make some pretty good discoveries, having the know how to poke around in the right places as you do. I will be patiently waiting to read about your progress. We need you, Tony.

Cheers,

Gil

Gil
Thanks for checking out the clips and joining the discussion..This amp has totally changed my view on having to use certain tubes dialing in the sweet spot on the trimmer and that type of compulsive behavior we sometimes obsess ourselves with.. If the fundamental property exists the rest is icing on an already sweet cake.. Wonderful idea on the loop inputs to determine where and hopefully what is contributing to this amps ability to take off.. I can say with some certainty that most of the feedback that is occurring is happening inside the amp and not so much as one being acoustically driven like most amps I have played.. I have to go out of town a few days but will do the experiment and will report my results to you..Yes I will crack this if it KILLS ME!!

All The Best...

Tony

Pre out of 102 variant into power amp section of 100w Music man
Gil
It would appear it is coming from the pre amp section.. This time no Dumbleator and it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!

http://www.youtube.com/watch?v=Lhm0j_ppkaQ

Tony

[ToneMerc]

it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!
Tony

I concur, you got some magic at the front end there.

TM

[Max]

It would appear it is coming from the pre amp section.. This time no Dumbleator and it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!

http://www.youtube.com/watch?v=Lhm0j_ppkaQ

Tony, IMO it might be interesting comparing this clip now with a clip recorded with exactly the same gear and setup (guitar, cable, mic, speaker, etc. etc.) but using the complete MM amp (MM preamp with the V1 and V2 tubes of your "Feedback City" amp and connected to the MM power amp by the same cable as the one by which your Feedback City preamp was connected to the MM power amp in this clip) setup in a way that its tone is as similar as possible to the one of your "Feedback City" amp in this clip.

Nice Sunday!

Max

[lovetone]

Hi Tony thats great progress, i wonder if there is more gain in the first stage may be due to a resistor thats higher in value, you might need to compare the gain of this amp with that of another. You could inject the same signal in to feedback city and then into another high plate amp and compare the results.

I would normally inject 40mV pk to pk into the input and see 1.0V pk to pk on the output of V1a with around 190V on the plate. That always seems to be my bench mark, that said I have never had that amount of bloom, I might ask if you would sell me tha amp when your ready.

Geoff

[talbany]

It would appear it is coming from the pre amp section.. This time no Dumbleator and it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!

http://www.youtube.com/watch?v=Lhm0j_ppkaQ

Tony, IMO it might be interesting comparing this clip now with a clip recorded with exactly the same gear and setup (guitar, cable, mic, speaker, etc. etc.) but using the complete MM amp (MM preamp with the V1 and V2 tubes of your "Feedback City" amp and connected to the MM power amp by the same cable as the one by which your Feedback City preamp was connected to the MM power amp in this clip) setup in a way that its tone is as similar as possible to the one of your "Feedback City" amp in this clip.

Nice Sunday!

Max

Max
Here is a quick clip of the MM with roughly the same volume and gain settings,,,Same guitar speakers cords etc.. The music man has it's spots where it feeds back where as the 102 variant doesn't seem to be as picky. They all pretty much take off!...Of coarse if I increase the gain on the MM it will search deeper for feedback!! The MM does have a throat-ier type voice and a little more hair, where as the 102 is smoother and a bit more focused.. The MM has a 250K drive and a classic stack no snubbers and no LNFB V1 and 100uf on the plates..KOA Speer MF on the plates and Cathodes..Phiers for grid resistors and Fender CTS Pots throughout..

http://www.youtube.com/watch?v=DTF91cfBGho

I would normally inject 40mV pk to pk into the input and see 1.0V pk to pk on the output of V1a with around 190V on the plate. That always seems to be my bench mark, that said I have never had that amount of bloom, I might ask if you would sell me tha amp when your ready.

Geoff
Thanks for the tip and thanks for the offer on the amp..However since some of the parts were donated to me with the intention of keeping the amp in the family, I must decline the offer..I would however be glad to build you another like this one if you wish!!..

Tony

[ayan]

Pre out of 102 variant into power amp section of 100w Music man
Gil
It would appear it is coming from the pre amp section.. This time no Dumbleator and it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!

http://www.youtube.com/watch?v=Lhm0j_ppkaQ

Tony

I'm just thinking out loud: to recap, the magic comes from the preamp, and since it happens in clean mode as well, one would think that whatever's going on is happening in V1. That is very intriguing, because there are not a lot of ways in which this could be happening.... namely, V1B' output may somehow be finding its way to V1A's grid. Other than that, I don't know what else to think. But if that's what's going on, it'd be strange in that the path to V1A's grid is very simple, a straight piece of shielded cable from the input jack to the grid. How could the the 2nd stage output ever find its way there? All the wiring to the front panel switches and pots carries a signal which is out of phase with the input, so I wouldn't think that plays much of a role. Maybe have a close look at the V1 socket? I simply don't know what to think.

PS: Just one crazy thought... you have the V1 filter cap old style, by the 1st tube socket. How close does the red B+ wire from the cap (going to the plate load resistors) run to V1A's grid? Can't tell from the picture.

Gil

[talbany]

Pre out of 102 variant into power amp section of 100w Music man
Gil
It would appear it is coming from the pre amp section.. This time no Dumbleator and it seems the bloom feedback sustain is a bit more intense using the MM output section..So much for the filtering huh!!

http://www.youtube.com/watch?v=Lhm0j_ppkaQ

Tony

I'm just thinking out loud: to recap, the magic comes from the preamp, and since it happens in clean mode as well, one would think that whatever's going on is happening in V1. That is very intriguing, because there are not a lot of ways in which this could be happening.... namely, V1B' output may somehow be finding its way to V1A's grid. Other than that, I don't know what else to think. But if that's what's going on, it'd be strange in that the path to V1A's grid is very simple, a straight piece of shielded cable from the input jack to the grid. How could the the 2nd stage output ever find its way there? All the wiring to the front panel switches and pots carries a signal which is out of phase with the input, so I wouldn't think that plays much of a role. Maybe have a close look at the V1 socket? I simply don't know what to think.

Gil

Gil
Thanks for getting back on this.. I agree and am a bit stumped myself.. Last week I took the amp to a friends house who has played an original Dumble several years ago( he was not sure of the model or type ODS) After playing mine he mentioned the clean channel acted very much the same way as the Dumble he played..(Feedback at will on clean channel with the PAB on) He said this was the thing he most remembered about the amp... There must be something we are overlooking here and I have a funny feeling it's right in front of us!!
Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!

Tony

[ayan]

Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!

Tony

Can't say the ones I tried did. However, I remember Brandon saying that LC's Dumble feedback like crazy on the clean channel...

I edited my last post and asked about the B+ red wire for V1, how close is that to V1A's grid?

Gil

[talbany]

Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!

Tony

Can't say the ones I tried did. However, I remember Brandon saying that LC's Dumble feedback like crazy on the clean channel...

I edited my last post and asked about the B+ red wire for V1, how close is that to V1A's grid?

Gil

I dont think it's close enough to cause any coupling!!

Tony

[brentm]

Have all the original Dumbles you have played have this ability to run away like 124?
Hey Max same question to you as well!!

Tony

Can't say the ones I tried did. However, I remember Brandon saying that LC's Dumble feedback like crazy on the clean channel...

I edited my last post and asked about the B+ red wire for V1, how close is that to V1A's grid?

Gil

I dont think it's close enough to cause any coupling!!




Tony

Is it possible to test/troubleshoot the feedback using an oscope? How was the PI eliminated as source of the feedback? This is getting very interesting!!!! You guys are really close to unlocking one of the dumble "component location" secrets!